1. Field of the Invention
The present invention relates to holographic display systems. More particularly, the present invention relates to holographic display systems wherein the light source illuminating the hologram for replay is in close proximity to the hologram, and the holographic image is light source modification suitable as a room illumination device in a dim light environment.
2. The Prior Art
Holograms are a well-known, visually fascinating, and entertaining way of producing 3-D holographic images for viewing. A hologram is a recording of information that can be illuminated with a beam of light to reconstruct or "replay" the holographic image recorded in the hologram. Improved detail and realism of holographic images have resulted in the application of holography to medicine, business, the arts, etc. when very detailed and accurate 3-D images are desired.
The sharpness of the image produced by a hologram is affected by the manner in which a hologram is illuminated. It is well known in the art that a laser light source typically provides illumination to a hologram to produce the sharpest image possible, but the cost and know-how required to implement the laser light source can be prohibitive. It is also known in the art to use an incandescent lamp or "white light" source for illuminating holographic image for replay.
With an incandescent source, however, the quality of the image produced depends particularly upon how accurately the reconstruction source matches the "reference source" used to record the holographic image in the hologram. As a consequence, without a good match, using an incandescent source can result in a blurred image during replay. One remedy to this problem, known in the art, is to place the incandescent source a long distance from the hologram so as to reduce the effective angle subtended by the light source at a point on the surface of the hologram. This is known to those of ordinary skill in the art as reducing the extent of the light source. In this manner, the incandescent source can be made to represent an approximation of a point light source.
It has been found, however, that placing the light source a long distance from the hologram is often times not a practical implementation. To overcome this practical defect, various arrangements of reflectors used to fold the light beam back on itself are known in the art. These known systems are often expensive and bulky, and as a result, are not useful for many of the applications to which holographic images can be put.
Another approach, disclosed in UK Patent Application No. 2 239 340, to overcoming blurring due to the extent of the light source which occurs in employing a white light source for illumination, without unduly increasing the distance between the source of illumination and the hologram, a linear light source was employed, wherein the linear light source extended substantially parallel to the surface plane of the hologram. The linear light source comprised either an elongated filament extending parallel to the plane of the hologram or a light source shining through an elongated slip or collimator.
Though this approach may have reduced the source extent blurring of the replayed image in the direction perpendicular to the length of the filament, a light source configured in this manner still blurs the replayed image in the direction parallel to the filament. Also, due to the intensity of the disclosed linear filament light source, the replayed holographic image would produce a bright illumination not suitable for use in low light environments, and the disclosed shielding would not substantially prevent reflected and scattered light from the linear light source from adding to the illumination of a hologram user's viewing area. Further, the light source enclosure does not prevent the scattering or reflecting of light towards the hologram. The scattered or reflected light reconstructs ghost images and effectively increases the extent of the light source to blur the desired image.